Possible Space-Inversion (P) and Time-Reversal (T) Symmetry Breaking in Poly(n-butyl-isobutylsilane) and Homologs

To probe a possibility of symmetry breaking within chainlike σ-conjugated polymers, we investigated the preservation of P-odd/T-even and P-odd/T-odd symmetries in semiflexible poly(n-butyl-isobutylsilane) (C4-iBS), which lacks point chirality. Using circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopy in various achiral solvents, we recorded three sets of temperature-dependent CD generation and inversion at the Siσ–Siσ* transitions, which were assigned to 114-, 83-, and 73-helices, respectively. In n-hexadecane-h34, C4-iBS exhibited a CD inversion due to an 83-helix reversal from gabs = −1.3 × 10−3 (40 °C) to +0.7 × 10−3 (20 °C). The observed H/D isotope effects in n-hexadecane-h34 and -d34 suggest that 1H and 2H nuclei contribute differently to the temperature-dependent 83-helicity. Remarkably, photoexcited C4-iBS at 20 °C displayed dynamic CPL oscillations at 330 nm, alternating between glum = −7 × 10−2 and +9 × 10−2 at ~20 s intervals. These CD and CPL results suggest a mixing of P-odd and P-even states in both S0- and S1-states. Furthermore, magnetic circular dichroism (MCD) and magnetochiral dichroism (MChD) under a reversing H0 = ±0.45 T at 23 °C produced non-mirror-image MCD/MChD spectra and their non-identical unpolarized MUV/MChUV spectra. These observations and analysis imply the simultaneous mixing of P-odd/P-even and T-odd/T-even terms in the S0- and S1-states of C4-iBS.